feat(altair): implement line-timeseries (#6119)

## Implementation: `line-timeseries` - python/altair

Implements the **python/altair** version of `line-timeseries`.

**File:** `plots/line-timeseries/implementations/python/altair.py`

**Parent Issue:** #2006

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/anyplot/actions/runs/25590385550)*

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Markus Neusinger <2921697+MarkusNeusinger@users.noreply.github.com>

Author: github-actions[bot]

plots/line-timeseries/implementations/python/altair.py

@@ -1,14 +1,29 @@
-""" pyplots.ai
+""" anyplot.ai
 line-timeseries: Time Series Line Plot
-Library: altair 6.0.0 | Python 3.13.11
-Quality: 92/100 | Created: 2025-12-26
+Library: altair 6.1.0 | Python 3.13.13
+Quality: 92/100 | Updated: 2026-05-09
 """
 
-import altair as alt
-import numpy as np
-import pandas as pd
+import os
+import sys
 
 
+# Remove the current directory from sys.path to avoid importing altair.py
+sys.path = [p for p in sys.path if p != "" and not p.endswith("python")]
+
+import altair as alt  # noqa: E402
+import numpy as np  # noqa: E402
+import pandas as pd  # noqa: E402
+
+
+# Theme tokens
+THEME = os.getenv("ANYPLOT_THEME", "light")
+PAGE_BG = "#FAF8F1" if THEME == "light" else "#1A1A17"
+ELEVATED_BG = "#FFFDF6" if THEME == "light" else "#242420"
+INK = "#1A1A17" if THEME == "light" else "#F0EFE8"
+INK_SOFT = "#4A4A44" if THEME == "light" else "#B8B7B0"
+BRAND = "#009E73"  # Okabe-Ito position 1
+
 # Data - Daily stock prices over one year
 np.random.seed(42)
 
@@ -29,41 +44,56 @@
 # Create time series line chart
 chart = (
     alt.Chart(df)
-    .mark_line(
-        strokeWidth=3,
-        color="#306998",  # Python Blue
-    )
+    .mark_line(strokeWidth=3, color=BRAND, point=True, size=150)
     .encode(
         x=alt.X(
             "date:T",
             title="Date",
             axis=alt.Axis(
-                format="%b %Y",  # Month Year format
+                format="%b %Y",
                 labelFontSize=18,
                 titleFontSize=22,
                 labelAngle=-45,
                 tickCount=12,
+                labelColor=INK_SOFT,
+                titleColor=INK,
+                domainColor=INK_SOFT,
+                gridColor=INK,
+                gridOpacity=0.10,
             ),
         ),
         y=alt.Y(
             "price:Q",
             title="Stock Price ($)",
             scale=alt.Scale(zero=False),
-            axis=alt.Axis(labelFontSize=18, titleFontSize=22, grid=True, gridOpacity=0.3),
+            axis=alt.Axis(
+                labelFontSize=18,
+                titleFontSize=22,
+                labelColor=INK_SOFT,
+                titleColor=INK,
+                domainColor=INK_SOFT,
+                gridColor=INK,
+                gridOpacity=0.10,
+            ),
         ),
         tooltip=[
             alt.Tooltip("date:T", title="Date", format="%B %d, %Y"),
             alt.Tooltip("price:Q", title="Price", format="$.2f"),
         ],
     )
     .properties(
-        width=1600, height=900, title=alt.Title("line-timeseries · altair · pyplots.ai", fontSize=28, anchor="middle")
+        width=1600,
+        height=900,
+        background=PAGE_BG,
+        title=alt.Title("line-timeseries · altair · anyplot.ai", fontSize=28, anchor="middle", color=INK),
     )
-    .configure_view(strokeWidth=0)
-    .configure_axis(gridColor="#cccccc", gridOpacity=0.3, domainColor="#333333")
+    .configure_view(fill=PAGE_BG, stroke=INK_SOFT, strokeWidth=1)
+    .configure_title(color=INK, fontSize=28)
+    .configure_legend(fillColor=ELEVATED_BG, strokeColor=INK_SOFT, labelColor=INK_SOFT, titleColor=INK)
     .interactive()
 )
 
 # Save outputs
-chart.save("plot.png", scale_factor=3.0)
-chart.save("plot.html")
+script_dir = os.path.dirname(os.path.abspath(__file__))
+chart.save(os.path.join(script_dir, f"plot-{THEME}.png"), scale_factor=3.0)
+chart.save(os.path.join(script_dir, f"plot-{THEME}.html"))